Interface engineering of Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 via in situ built LiI/ZnLi x mixed buffer layer for solid-state lithium metal batteries.

Garnet-type solid-state Li metal batteries (SSLMBs) are viewed as hopeful next-generation batteries due to their high energy density and safety. However, the major obstacle to the development of garnet-type SSLMBs is the lithiophobicity of Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO), resulting in a large interfacial impedance. Herein, a LiI/ZnLi x mixed ion/electron conductive buffer layer is constructed at the interface by an in situ reaction of molten Li metal with ZnI 2 film. This mixed buffer layer ensures close contact between the Li metal and garnet, significantly reducing interfacial impedance. As a result, the Li symmetrical cell with the LiI/ZnLi x buffer layer shows an interface impedance of 10.3 Ω cm 2 , much lower than that of the cell with bare LLZTO (1173.4 Ω cm 2 ). The critical current density (CCD) is up to 2.3 mA cm -2 , and the symmetric cells present a long cycle life of 2000 h at 0.1 mA cm -2 and 800 h at 1.0 mA cm -2 . In addition, the full cells assembled with the LiFePO 4 cathode show a capacity of 143.9 mA h g -1 after 200 cycles at 0.5C with a low-capacity decay of 0.021% per cycle. This work reveals a simple, feasible, and practical interface modification strategy for solid-state Li metal batteries.